In chasing threads, just as in other cutting operations, it is desirable to break each chip into small pieces so that the chip will not be wound around the cutting tool or workpiece. However, under conditions in which a depth of cut is extremely small and a feed rate is high, a chip does not readily break, and trying to forcibly break the chip by a chip breaker or the like, increases the cutting resistance undesirably. As a result, if the cutting tool or thread cutting insert is made of a material having an inferior rigidity, the cutting tool may be subjected to abnormal vibration, which lowers the machining accuracy.
For this reason, a conventional throw-away thread cutting insert 51, as shown in FIG. 1, either has no chip breaker or has a groove 53 provided near each cutting tip 52 in a manner to separate the cutting edge 52 from the insert body. However, in such a throw-away insert 51, it has been impossible to break a chip into small pieces, to control the direction of guide and removal of the chip or to stabilize the curling of the chip. Thus, the elongated chip undesirably winds around the cutting tool, the workpiece or the like.
In view of the above difficulty and problem accompanying the conventional thread cutting insert, the present invention is a throw-away thread cutting insert in which a recess if formed in a rake surface so as to smoothly remove chips as disclosed in Laid-open Japanese Utility Model Application Nos. 63-113502 and 64-12705 (both unexamined).
However, in the throw-away insert disclosed in JUMA No. 63-113502, the recess is disposed rearwardly of the cutting tip at a distance therefrom and, therefore, the leading of chips from the cutting edge to the recess cannot be satisfactorily controlled. In the case of the throw-away insert disclosed in JUMA No. 64-12705, the recess is disposed near the cutting tip and, accordingly, although chips can be smoothly guided and led to the recess, the chips cannot satisfactorily be curled into a helical shape (i.e., a regular curl-shape) at the recess. Thus, the proposed thread cutting inserts having a recess still raise problems.